As compared to conventional amorphous silicon thin film transistors, metal oxide or metal oxynitride thin film transistors have many unique advantages such as a higher carrier density and a higher mobility rate. Accordingly, the metal oxide or metal oxynitride thin film transistors can be made smaller, and the display panel made of such thin film transistors can achieve a higher resolution and a better display effect. The metal oxide or metal oxynitride thin film transistors have found a wide range of applications in display field. The metal oxide or metal oxynitride thin film transistors, however, typically require an etch stop layer on the active layer, otherwise degraded transistor reliability will occur. Fabrication of the etch stop layer requires an additional patterning process using an additional mask plate, increasing manufacturing costs. Attempts to reduce the number of patterning processes, for example, using a half-tone mask plate, often lead to further complications not easily controllable, affecting product quality.